Various materials have been used as charge generating materials in electrophotographic photoreceptors in the past. Charge generating materials can be broadly classified as inorganic pigments, such as selenium, zinc oxide or cadmium sulfide, or organic pigments.
A variety of organic pigments are used in the so-called organic photoreceptor, in which the organic pigments are used in electrophotographic photoreceptors. Typical examples of the organic pigments include the polycyclic quinone-based pigments, such as dibromoanthoanthrone, dibenzylpyrenequinone pigments, pyranthrone pigments, perylene pigments, and phthalocyanine-based pigments, such as non-metallic phthalocyanines, vanadium phthalocyanine, copper phthalocyanine etc. (For example, see JP-A-47-18544) (the term "JP-A" as used herein means an "unexamined published Japanese patent application.")
The grain size of an organic pigment is reduced and rendered uniform when the pigment used in the electrophotographic photoreceptor is subjected to an acid pasting treatment in which the pigment is reprecipitated by dilution with water and this technique has already been proposed as a way of providing electrophotographic photoreceptors which have good electrophotographic characteristics (see JP-A-62-206558).
However, differences arise in the chargeabilities and dark decay coefficient of the electrophotographic photoreceptors obtained when such pigments are used in electrophotographic photoreceptors, even when the photosensitive layers are formed using the same type of organic pigment. Therefore, the electrophotographic characteristics of the products are variable and suffer from the disadvantage that products which have low chargeability and a high dark decay coefficient are often obtained.